It is well-known in the art that amination of 4,5-dichloro-2-phenyl-3(2H)-pyridazinone (PCC) leads to the formation of so-called commercial Pyrazon, that is a mixture of two isomers, one of which, 5-amino-4-chloro-2-phenyl-3(2H)-pyridazinone (PCA) is active as a selective weed-killer or herbicide, while the second, 4-amino-5-chloro-2-phenyl-3(2H)-pyridazinone (ISO-PCA), is inactive as a weed-killer.